Acoustic wave devices using piezoelectric thin film resonators are used as filters and duplexers of wireless devices such as, for example, mobile phones. The piezoelectric thin film resonator has a structure designed to have a lower electrode and an upper electrode facing each other across a piezoelectric film. The region where the lower electrode and the upper electrode face each other across the piezoelectric film is a resonance region.
Rapid diffusion of wireless systems has promoted the use of many frequency bands. As a result, filters and duplexers have been required to have steeper skirt characteristics. One way of making the skirt characteristic steeper is to increase the Q-value of the piezoelectric thin film resonator.
Japanese Patent Application Publication No. 2006-109472 (Patent Document 1) discloses a piezoelectric thin film resonator including an anulus located on the surface of one of the upper electrode and the lower electrode. Japanese Patent Application Publication No. 2014-161001 (Patent Document 2) discloses a piezoelectric thin film resonator in which an insertion film is inserted into the piezoelectric film in the outer peripheral region of the resonance region. U.S. Pat. No. 9,048,812 (Patent Document 3) discloses a piezoelectric thin film resonator including an anulus, called a bridge, in the piezoelectric film.
In the piezoelectric thin film resonators disclosed in Patent Documents 1 through 3, the leak of an acoustic wave energy from the resonance region can be reduced, and the Q-value can be improved. However, it is difficult to sufficiently reduce the leak of the acoustic wave energy from the resonance region, and the degree of the improvement of the Q-value is insufficient.